The present invention relates generally to knee prostheses and, more particularly, to a resurfacing type of total knee prosthesis which also provides a posterior stabilization function over the entire range of flexion, and which in one embodiment includes a hinged construction.
Known total knee prostheses can essentially be classified into three basic categories. In the first category, the articular surface of the distal femur and proximal tibia are "resurfaced" with respective metal and plastic condylar-type articular bearing components. These knee prostheses provide adequate rotational and translational freedom and require minimal bone resection to accommodate the components within the boundaries of the available joint space. The patella-femoral joint may also be resurfaced by a third prosthetic component, as well. The femoral, tibial and patella prosthetic resurfacing components are affixed to respective, surgically prepared adjacent bone structure by a cementing or by a biological bone ingrowth fixation means.
The femoral component is a metallic alloy construction (cobalt-chrome alloy or 6Al4V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint. The tibial component can be made entirely of plastic (UHMWPE: ultra high molecular weight polyethylene) or can be comprised of a metallic base component, distally, and an interlocking plastic (UHMWPE) component, proximally. The plastic tibial plateau bearing surfaces are of concave multi-radius geometry to more or less match the articular geometry of the mating femoral condyles, depending upon the desired design mechanics of primary femoro-tibial motion, e.g. the flexion-extension mode-including posterior rollback and the secondary rotational and translational articular motions. In the resurfacing type of total knee prostheses both the femoral and tibial components are positioned on the respective side of the knee joint and are not mechanically connected or linked together, as in the case of constrained or hinged type of knee prostheses, which constitutes another or second category of total knee prostheses.
Additionally, in resurfacing types of total knee prostheses according to the first category, as previously stated, the tibial plateau bearing surface geometry can assume a variety of configurations, depending upon the desired extent of articular contact congruency and associated translational (medial-lateral and anterior-posterior) and rotational (axial and varus-valgus) secondary femoro-tibial motions. These various secondary motions allow the resurfaced knee to function in a natural-like biomechanical manner in conjunction with the surrounding ligamentous and muscle structures about the knee joint. The viable soft tissue structures functionally maintain the femoral and tibial bearing surfaces in contact, provide the necessary levels of constraining force to achieve knee joint stability, and decelerate the principal motion in flexion-extension and secondary motions, such as axial rotation, etc. in a controlled manner. Additionally, this functional interaction between the surrounding tissue structures and the implanted knee prosthesis minimizes abrupt motion stoppage or impact loading of properly designed prosthetic articular surfaces, and thus prevents overstressing at the component fixation interface. Examples of resurfacing types of total knee prosthetic devices are disclosed in U.S. Pat. Nos. 3,774,244 to Walker; 3,728,742 to Averill et al.; 4,081,866 to Upshaw et al. and 4,207,627 to Cloutier.
On the other hand, the mechanically linked, constrained or hinged type of knee prosthesis according to the second category provides a fixed fulcrum flexion-extension capability. Some of these devices are fully constrained in axial rotation, while others provide either partially constrained or unconstrained axial rotational freedom. The "hinged knee" therefore, is usually surgically indicated in selected cases where the surrounding soft tissue structures are grossly degenerated and incapable of providing functionally acceptable knee joint stability. Examples of this type of total knee prosthetic device are disclosed in U.S. Pat. Nos. 3,996,624 to Noiles; 3,837,009 to Walker; and 4,136,405 to Pastrick et al.
In clinical situations where prosthetic knee joint reconstruction is surgically indicated in the presence of compromised posterior (tibia-to-femur) stability, that is, due to absent or incompetent posterior cruciate ligament structures, a posterior-stabilized total knee device can be utilized. This type of device constitutes the third category of total knee prosthetic devices. The posterior-stabilized total knee device essentially incorporates all of the functional features of the first category, that is, the resurfacing condylar-type of knee prostheses, in addition to incorporating a mechanical cam/follower mechanism for providing posterior (tibia-to-femur) constraint. The cam/follower mechanism is almost universally positioned within the intercondylar space of the femoral component and provides substitutional posterior constraint, as a predesigned compensation feature for lost posterior cruciate function or for compromised posterior knee stability. Thus, the possibility of anterior dislocation of the femur is reduced.
Additionally, the cam/follower mechanism is generally positioned between the anterior and posterior femoral condyles and dimensionally spans the femoro-tibial joint space; protruding into the sector of intercondylar bone within the distal femur; thus, occupying the sector within the knee joint where the cruciate ligaments are anatomically located. Surgical selection of a posterior stabilized type of knee prosthesis, therefore, generally predisposes the use of this type device exclusively in clinical situations where the natural posterior (and anterior) cruciate ligament structures of the knee joint are absent or sacrificed.
The cam portion of the cam/follower mechanism of the posterior stabilized device generally includes a convex lobe-shaped surface, integrally machined or cast within a box-like structure known as the "stabilizer box", which in turn, is integrally located between the medial and lateral condyle runners of the femoral component and positioned between the anterior and posterior condyles. The cam surface is generally formed within the posterior wall portion of the stabilizer box and is bounded by the superior wall on the top, the medial and lateral wall portions on the sides and the anterior wall portion at the front. The stabilizer box structure, thus formed, occupies a significant envelope, relative to the overall dimensions of the femoral component and therefore, requires a substantial resection of viable bone to allow its accommodation within the intercondylar sector of the distal femur.
The articular surface of the posteriorly positioned and anteriorly oriented convex cam member is generally precisely ground and highly polished. The convex cam articulates with the anteriorly positioned and posteriorly oriented follower member, as the knee undergoes femoro-tibial flexion and extension articulation. The mating follower surface is machined integral within the ultra-high molecular weight polyethylene (UHMWPE) tibial plateau bearing component. The follower member usually consists of a relatively concave articular surface located on the posterior side of an upwardly extending post-like structure, which is positioned between the concave medial and lateral tibial plateau bearing surfaces.
The post-like structure of the follower member extends upward and generally is of sufficient over-all height to span the joint space; thus, protruding into and occupying an appropriate position within the stabilizer box portion for mating with the cam member of the cam/follower mechanism. The cam/follower mechanism, therefore, not only functionally compensates for lost posterior cruciate function, but also occupies essentially the same intercondylar position within the knee joint; thus requiring that the posterior (and anterior) cruciate ligament structures be either absent or sacrificed.
The resultant action of the contacting cam/follower mechanism, thus described, provides posterior stabilization or constraint of the tibial component, relative to the femoral component, generally from about mid-range to full range of flexion. Within this limited range, therefore, the posterior stabilizing mechanism essentially simulates the functional contribution of the natural posterior cruciate ligaments attached between the anterior femur and posterior tibia aspects of the knee joint. Additionally, since the cam/follower surface geometry is generally non-congruent, the mechanism can be designed to produce posterior femoro-tibial rollback, simulating the biomechanical kinematic displacement characteristics of the natural knee joint.
Examples of posterior-stabilized total knee prostheses of the type just described are disclosed in U.S. Pat. Nos. 4,209,861 to Walker; 4,298,992 to Burstein et al.; 4,213,209 to Insall et al; and 4,888,021 to Forte et al. Each of the devices described in the above patents incorporates a UHMWPE tibial plateau bearing component with a pair of medial and lateral concave bearing surfaces, and a metal alloy femoral component with mating multi-radius condylar runners which ride on the bearing surfaces. In all cases prosthesis surface geometries approximate the articular surfaces of the natural knee. The articulation of the femoral condyles with the tibial plateau bearing surfaces allows primary femoro-tibial flexion and extension, and secondary motions of axial and varus-valgus rotations and anterior-posterior and medial-lateral translations. The knee joint reaction forces during primary or secondary motion are principally supported by the tibial bearing surfaces, and to some extent by the cam/follower surfaces, and are transferred to the underlying fixation interfaces and adjacent supportive bone structures.
Additionally, the above referenced designs incorporate a UHMWPE tibial bearing component with an upwardly extending post-like follower structure, which is positioned between the plateau bearing surfaces, slightly anterior of the component mid-line. The generally concave follower surface is integrally machined on the posterior side of the central post structure. With the femoral and tibial knee components in a normally reduced, surgically implanted position, the upwardly extending tibial post protrudes into the stabilizer box structure located within the intercondylar space of the femoral component. Posterior tibial constraint is achieved when the posteriorly oriented concave face of the follower contacts the generally anteriorly oriented convex lobe surface of the cam.
Both cam/follower articulation and femoro-tibial articulation occur concurrently during knee flexion-extension. The commencement of cam/follower contact, and hence, commencement of posterior stabilization occurs on or about mid-flexion range for the devices described in U.S. Pat. Nos. 4,213,209 and 4,298,992, and near the onset of knee flexion for the devices described in U.S. Pat. Nos. 4,888,021 and 4,209,861. It should be noted that however, that there are forces acting over the entire range of motion of the patented devices that are not accounted for in the patent disclosures.
In each of the above devices, the existence of a relatively large stabilizer box at the mid-portion of the femoral component requires resection of a significant block of viable intercondylar bone to accommodate the implantation of the femoral component prosthetic device. Moreover, additional surgical instrumentation is required and the surgical procedure is somewhat more complicated compared to a conventional condylar-type knee resurfacing device. Additionally, the cam/follower mechanism of the referenced devices essentially occupies the same intercondylar position within the knee joint as the cruciate ligaments; therefore, the use of these posterior stabilized knee prostheses are usually surgically indicated in those clinical situations where the cruciate ligaments are either absent or incompetent, or such ligaments must be intentionally sacrificed for these prostheses to be implanted. In other words, these posterior stabilized knee prostheses generally can not be effectively employed to function in conjunction with retained, viable posterior (and anterior) cruciate ligament structures because of space limitations and impending destructive interference with the cam/follower structure.
Furthermore, in each instance, the stabilizer box member has prominent (high profile) medial and lateral side walls and also, anterior and posterior walls. These bounding surfaces can inadvertently contact the upwardly extending tibial post during severe excursions of secondary knee motion, that is, during axial and varus-valgus rotations and medial-lateral translation; hence, can function to constrain these secondary movements within certain limitations, commensurate with design dimensional clearance. While these devices have been effective in constraining these types of motion excursions and therefore, effective in providing a high degree of controlled femoro-tibial stability, the resulting force reactions occurring between the stabilizer box surfaces and tibial post can produce periodic and severe (moment and torque) loading at the tibial component fixation sub-structure interfaces, which can cause complications related to component loosening.
Another type of posterior stabilized knee prosthesis is described in U.S. Pat. Nos. 4,892,547 and 4,959,071 to Brown. These designs incorporate a cam/follower mechanism having a low profile and require no resection penalty associated with the accommodation of a protruding stabilizer box structure. The heights of the tibial follower post (eminence) and the femoral cam members are no greater than the thickness of the distal and posterior condyles of the femoral component. The required femoral resections are thus identical to those of a conventional resurfacing condylar type of knee prosthesis of similar size and geometric design.
The devices disclosed in these patents however, suffer from other disadvantages. First, although the relatively short extending tibial post is located between the plateau bearing surfaces, the cam member is integrally incorporated at a high position, between the posterior condyles of the femoral component. These knee devices are, therefore, described as "partially stabilized" knee joint prostheses; since, the cam/follower mechanism only comes into contact after flexion of approximately 40 degrees has occurred and continues until full flexion is attained. Thus, there is no posterior tibia-to-femur constraint of the reconstructed knee joint from the outset of flexion to approximately 40 degrees flexion. In this regard, these knee devices are functionally similar to other posterior stabilized knee devices, such as those disclosed in U.S. Pat. Nos. 4,213,209 and 4,298,992, previously discussed. These patents, therefore, do not take into account the forces acting over the entire range of motion of the knee joint.
Further with respect to the referenced patents to Brown, when the follower first contacts the cam surface at approximately 40 degrees flexion, mechanical posterior rollback is initiated due to the differences in articular curvature. In a normal knee, physiological femoro-tibial rollback starts at the onset of knee flexion and is generally mostly completed by 40 degrees of flexion. This rollback is believed to provide substantially a purely rolling motion of the condyles on the tibial plateau bearing surfaces (femoro-tibial motion), after which there is a transitional motion of rolling and sliding. Therefore, it is desirable that the beginning of cam/follower contact for initiation of the posterior rollback phase of knee motion occurs as early as possible in the flexion range, and also that completion of rollback mostly occurs at or preferably before approximately 40 degrees of flexion is experienced. The '547 and '071 patents furthermore describe the action of the cam/follower mechanism of the disclosed devices, in producing posterior rollback, simulating the rolling-type of posterior displacement of the femoro-tibial articular contact in the natural knee during knee flexion. However, the indicated commencement of this roll-back feature in the '547 and '071 patents "begins after flexion of the knee joint through approximately 40 degrees (flexion), and ends after flexion of the joint through approximately 90 degrees (flexion)." In the normal knee, through the complex active interaction of the anterior and posterior cruciate ligaments and other surrounding adjacent soft tissue structures, the rollback phase of femoro-tibial articulation commences early in the flexion range, as aforementioned, and is essentially mostly completed at 40 degrees flexion; with the character of the primary articulation, after completion of posterior rollback, gradually changing to a gliding and then sliding mode in a manner approaching that of a fixed fulcrum posterior condyle rotation.
Third, the geometry or shape of the articular surface of the cam and follower members in the '547 and '071 patents are not described as being congruent, and therefore, the functional contact area is small and the resultant contact stresses are high when joint loading which tends to produce anterior dislocation of the femur in imposed. Articular surface congruence of the cam and follower member is incorporated in the posterior stabilized device described in U.S. Pat. No. 4,888,021. This latter patent utilizes the aforementioned box-like "stabilizer" structure and therefore, requires a large resection of intercondylar femur bone to accommodate accurate seating of the femoral component. The relative advantages of large bearing contact surface, associated with joint prosthesis bearing surface congruency is well known and has been adopted in a number of prosthetic devices for the knee joint, as well as for the other joints of the human body, e.g. the shoulder and hip.
Fourth, the referenced cam and follower members, although of limited height to prevent intrusion within the intercondylar bone space of the distal femur, still occupy an intercondylar position of the reconstructed knee joint which, like the other posterior stabilizing knee prostheses previously referenced and described, generally are principally prescribed in surgical situations where the cruciate ligaments are absent, non-viable and/or are intentionally sacrificed.
In copending application Ser. No. 07/673,790 filed Mar. 22, 1991 by the present inventor, a posterior stabilized knee prosthesis construction is disclosed which distinguished the known prior art in many respects, among them by the provision for cam follower posterior stabilization inboard of the condyles and attempts by such construction to simulate normal knee movement and femoral/tibial interaction. Specifically, the condyles of the respective components rotate and the tendency for the femoral component to move anteriorly during full flexion is prevented by the cam/follower and support.
This construction, however, possesses a drawback, as the cam support and posterior stabilization is disposed inboard of the condyles and thereby cuts off the ability to retain any of the original ligamenture in the instance where surgical removal of less radical nature is possible. A need therefore exists for a posterior stabilized knee construction that can accommodate the retention of certain of the original joint structures while providing the stability, support and longevity of the natural knee.
With regard to hinged knee prostheses, mechanically linked or hinged type of knee prostheses generally provide a fixed fulcrum or uni-axis flexion-extension capability. Some of these devices are fully constrained in axial rotation, while others provide either partially constrained or unconstrained axial rotational freedom. Hinged knee prostheses are surgically indicated in selective cases involving gross and unreconcilabie knee joint instability; resulting from major destruction and incompetency of the surrounding soft tissue structures due to previous surgical failure, trauma, disease and congenital related conditions. Mechanical linking of the femoral and tibial prosthetic components by means of a hinge pin connection or some other bearing/connection means functionally compensates for loss of biomechanical knee constrainment and stability; which, is normally provided by the soft-tissue structures surrounding the knee joint.
Examples of uni-axis hinged type knee prosthetic devices are disclosed in U.S. Pat. No. 3,996,624 to Noiles and U.S. Pat. No. 4,136,405 to Pastrick et al. The Noiles and Pastrick et al inventions additionally provide partially constrained femoro-tibial axial rotation (rotating platform concept) with minimal medial-lateral or varus-valgus rotation. Further, the prosthesis disclosed in U.S. Pat. No. 3,837,009 to Walker provides variable axis flexion/extension motion but within the vertical or coronal plane of the knee joint and in a manner which does not involve natural-like posterior femoro-tibial rollback. A vertically oriented tear-drop shape slotted hole in the upwardly extending tibial post provides only vertical displacement of the femoro-tibial instant center, as a function of the multi-radius shape of the femoral condyles. Distal divergence of the vertical slotted hole provides radial clearance for the transverse hinge pin to allow slight axial rotational freedom and slight anterior-posterior freedom, over the latter stages of knee flexion. No provisions are incorporated in the Walker design to produce natural-like femoro-tibial posterior rollback nor posterior and anterior stabilization of knee joint motion.
It is therefore apparent that even hinged constructions suffer from the inability to simulate natural motion of the tibia and femur with respect to each other throughout the full range of knee motion encountered in everyday activity, and that a need therefore exists for a hinged construction that remedies the aforenoted deficiencies.